Both of these films are worth seeing. Of the two, I prefer "True Crime." It's thematically richer and more interesting to me, and I think it has a lot of thought-provoking social commentary about the death penalty that was missed upon its release.
In contrast,"Space Cowboys" is fun, but I think it's tonally a bit inconsistent. It starts off being a pretty effective buddy comedy about dealing with aging and remaining relevant in old age, but quickly devolves into a space drama in the second half of the film. I think the first half of the movie is more successful than the second half is, but I'd recommend both of these films. They are both entertaining.